HOMOTYPIC ADHESION OF RAT B-CELLS, BUT NOT T-CELLS, IN RESPONSE TO CROSS-LINKING OF CD48

Rat lymphocytes were found to aggregate in response to monoclonal anti bodies to the glycosyl phosphatidylinositol (GPI)-anchored surface ant igen CD48. This clustering required bivalent antibodies but was not Fc mediated. It was blocked by inhibitors of cellular metabolism and cyt oskeletal function but not by antibodies to leucocyte function-associa ted antigen-1 (LFA-1) or intracellular adhesion molecule-1 (ICAM-1). T he clusters were found to be due to homotypic adhesion of B cells, wit h T cells showing no response despite expressing equal levels of CD48. in addition, thymocytes, which are known to cluster in response to cr oss-linking of Thy-1, another GPI-anchored molecule, were found not to respond to cross-linking of CD48. These results suggest that specific signalling through CD48 in B cells, but not T cells, and through Thy- 1, but not CD48, in thymocytes, lead to cell adhesion events. This dif ferential signalling is interesting as neither CD48 nor Thy-1 have tra nsmembrane or intracellular domains. Levels of CD48-associated protein kinase activity were very low in both B and T cells, and no differenc e in the susceptibility to cleavage with phosphatidylinositol-specific phospholipase C was detected between B- and T-cell CD48.